Most dictionaries I checked had a definition similar to the one you found:
turnover (n.) the rate at which people leave and are replaced
That's what I found at Lexico and Collins, and Macmillan reads similarly:
turnover (n.) the rate at which people leave a place and new people arrive
However, perhaps informally over time, I suspect that people started to use the word to refer to the people themselves, rather than strictly as the rate, so that it would be acceptable in business circles to say something like, "We had seven turnovers last month," meaning seven people left the company. Indeed, I did find this definition in a business dictionary:
Human resource management: The number of employees hired to replace
those who left or were fired during a 12 month period.
Whether or not such usage would be appropriate in a news article or research paper might depend on how pedantic the intended audience might be. Your search showed there is some precedent, so you wouldn't be the first to use the word that way – but it might be safer to use the word only when referring to a rate.